The present invention pertains to a heater for mobile units, e.g., motor vehicles, comprising a burner for liquid fuel, which has a combustion air blower with a motor, a mixing device for producing a fuel-air mixture, an igniting electrode arrangement, and a combustion chamber, and a control device for controlling the ignition process.
Such heaters are usually used as so-called auxiliary or space heaters in passenger cars, trucks, and buses. The liquid fuel (gasoline or diesel fuel) is mixed with combustion air by means of a blower motor and a mixing device, and burned in a combustion chamber. The heat generated in the combustion chamber is removed by means of a heat carrier or heat transfer medium (air or water).
In certain devices, the fuel is atomized by means of a pressure atomizer. The fuel is expelled from the fuel nozzle by means of pressure, and air is admitted by the combustion air blower into the zone of the nozzle in order to thoroughly mix the fuel sprayed out with the air.
Good mixing of the air with the fuel has been known to be a requirement for good combustion. In heaters of the class being discussed here, the combustion chambers are relatively small because the most compact design possible is required, so that the mixing of the air with the fuel must take place within a narrowly limited space. It has now been observed that good mixing of air with the fuel in combustion chambers of small volume can practically be achieved only at relatively high air flow velocities. The combustion air blower therefore operates at a relatively high speed in such a heater.
Even though the high air flow velocity leads to good mixing of the air with the fuel during the operation, it has the disadvantage that ignition becomes difficult, especially at high operating voltages in a cold environment.
To improve the ignition properties of such a heater, one might propose the possibility of providing the blower motor with a protective resistor, which provides for a lower blower motor speed for a certain time period during the ignition phase. Aside from the electrical power loss associated with such an arrangement and the need to dissipate the heat generated in the protective resistor, it is difficult to set a speed suitable for the ignition process because of the inevitable speed tolerance.
One may also consider the possibility of designing an air supply passage such that a lower flow velocity is reached. However, such design solutions are risky, because there is a risk of excessive soot formation, especially at low voltage, because of poor mixing of the air with the fuel.